Jonny Lee Miller: How Running 50 Miles Can Save a Child's Life

On Saturday, the Elementary star will wake up in darkness and begin a 50-mile run through Bear Mountain, New York, on behalf of Jonah’s Just Begun, a non-profit formed by one of Elementary‘s crew members whose child was diagnosed with Sanfilippo.

Miller’s goal is to raise funds and awareness for the ultra rare disease, which can cause brain damage, bone deformities and organ failure, and often dramatically shortens the lives of children – many of whom don’t live beyond their teenage years.

“There’s not enough being done for these kids right now,” Miller, 42, tells PEOPLE.

So he runs with an incredibly powerful motivator: “They’re little kids, and they’re families, and what’s more important than that?”

Happily for Miller, his hobby also happens to be heroic: “I’m lucky to be able to do this,” he says. “I love running. What I’m trying to do in a small way is try and eke what benefit I can out of a small thing.”

He notes, “This is the way we often find ourselves helping people out – [the opportunity is] right there in front of you, and you say, ‘I can do this right now, this is how I can contribute in my own way.’ ”

An avid runner and sports enthusiast, Miller completed several marathons before escalating his training to tackle a 50-mile ultramarathon in 2013 because he was “really interested in what humans are capable of and what I was capable of – pushing, testing myself.”

Miller, who has also finished multiple 100-miles races for Jonah’s Just Begun, says mental commitment is his secret weapon as a runner. “You decide that you’re going to finish. Once you’ve done that, there’s nothing really that can stop you. … Once you’re at the start line, really, it’s over.”

While he’s raised nearly $200,000 for this race alone, Miller is most proud of the personal side of his efforts: “One impact is the families just realizing that somebody’s listening. It can mean a great deal to the families who are coping with this. It’s a devastatingly bleak situation to find yourself in where there’s no cure for your child, their life expectancy is short and there seems to be nothing happening.”

By running and speaking to Congress, he hopes “to fuel the fire of that research and keep money coming in.”

After he finishes one marathon this weekend, Miller has a little more ground to cover professionally as Elementary heads toward its season 3 finish line on May 14.

He offered this tease for Sherlock Holmes: “The biggest challenge [is that] he faces someone from his past whom he really dislikes who has a grip on him. His dark past rears its ugly head, and he has to really face that head-on in order to help a friend of his. He’s faced with some difficult personal choices.”

Elementary airs Thursdays (10 p.m. ET) on CBS. Donations for Jonah’s Just Begun can be made here.